In recent years a great requirement for smaller and smaller space-saving, light and economical display modules and displays for the rapid and adequate visualization of data has developed. Currently, LCDs (Liquid Crystal Displays) predominate in the area of flat screens for notebooks, mobile telephones and digital camera. However, they have a few disadvantages such as the strong angular dependency of contrast and colors, slow reaction times for image and contrast change and a low efficiency conditioned by a plurality of filters and polarizers, so that in order to achieve the required luminosity relatively high energies must be used. To this extent the demand for small, high-resolution, colored and current-saving screens with improved display qualities is great. Displays on the basis of organic light emitting diodes (OLEDs) represent an alternative to LCDs since they consist themselves of light-emitting pixels and therefore have no background illumination. They can be produced flexibly and thin with low production costs, e.g., in the form of a foil, and be operated with a relatively low expenditure of energy. Given their low operating voltage, the high energy efficiency as well as the possibility of manufacturing areally emitting components for the emission of any colors, OLEDs are also suitable for use in illuminating elements.
OLEDs are based on the principle of electroluminescence in which electron-hole pairs, so-called excitons, recombine under the emission of light. To this end the OLED is constructed in the form of a sandwich structure wherein at least one organic film is arranged as active material between two electrodes, positive and negative charge carriers are injected into the organic material and a charge transport takes place from holes or electrons to a recombination zone in the organic layer where a recombination of the charge carrier to singlet and/or triplet excitons occurs under the emission of light. The subsequent radiant recombination of excitons causes the emission of the visible useful light emitting by the light-emitting diode. In order that this light can leave the component at least one of the electrodes must be transparent. As a rule this transparent electrode consists of conductive oxides designated as TCOs (transparent conductive oxides). The starting point in the manufacture of an OLED is a substrate on which the individual layers of the OLED are applied. If the electrode nearest to the substrate is transparent the component is designated as a “bottom-emission OLED” and if the other electrode is designed to be transparent the component is designated as a “top-emission OLED”. The same applies to the case of completely transparent OLEDs, in which the electrode between the substrate and the at least one organic layer as well as the electrode at a distance from the substrate are designed to be transparent.
As explained, the generation of light in the active zone or emission zone of the component by radiant recombination of electrons and defect electrons (holes) takes place via excitonic states. The different layers of OLEDs, e.g., the transparent electrodes and the at least one organic layer have in general a different refractive index that is by nature greater than 1. To this extent not all generated photons can leave the component and be perceived as light since total reflections can occur on the different boundary surfaces within the component or between the component and the air. Furthermore, even a part of the generated light is reabsorbed within the component. Depending on the configuration of the OLEDs, a formation of optical substrate- and/or organic modes (that is, diffusion of light in the substrate, the transparent electrode and/or the at least one organic layer) takes place in addition to the diffusion of external modes based on the previously described total reflection. If the electrode nearest to the substrate is not transparent (top-emission OLED), in addition to external modes only modes in the at least one organic layer and/or the electrode at a distance from the substrate can diffuse that are designated in common as organic modes. Only the external optical modes can be perceived as light by the observer, whose proportion of the entire luminescence generated within the component is less than 20%, as a function of the configuration of the OLED. To this extent there is a need to decouple these internal optical modes, that is, organic-and optionally substrate modes more strongly from the component in order to achieve the highest possible degree of efficiency of the organic light-emitting component.
In order to improve the decoupling efficiency a plurality of methods and designs, in particular for bottom-emitting OLEDs, are known that concern the decoupling of optical substrate modes. To this end the article “30% external quantum efficiency from surface textured, thin-film light-emitting diodes” by I. Schnitzer, Appl. Phys. Lett., vol. 63, page 2174 (1993) suggests roughening the surface of the substrate, which avoids to a considerable extent the occurrence of total reflection on the boundary surface between substrate and air. This roughening can be achieved, e.g., by etching or sandblasting the substrate surface facing away from the organic. In the contribution “Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification” by C. F. Madigan, Appl. Phys. Lett., vol. 76, page 1650 (2000) the application of a spherical pattern on the back side of the substrate surface is described. This pattern can comprise, e.g., an array of lenses that is applied onto the substrate by pasting or laminating. The article “Organic light emitting device with an ordered monolayer of silica microspheres as a scattering medium” by T. Yamasaki et al., Appl. Phys. Lett., volume 76, page 1243 (2000) suggests applying microspheres of quartz glass onto the surface of the substrate in order to improve the decoupling of the light in an OLED. These microspheres can also be arranged adjacent to the OLED. Furthermore, it is also known that periodic structures with a period length in a range of the wavelength of the light emitted by the OLED can be generated between the substrate and the first electrode, the periodic structure being propagated into the optically effective layer of the light-emitting diode. The indicated geometry finally has a Bragg scattering as a consequence that increases the efficiency of the component, see J. M. Lupton et al., Appl. Phys. Lett., vol. 77, page 3340 (2000). Furthermore, German unexamined publication (Offenlegungsschrift) DE 101 64 016 A1 relates to an organic light-emitting diode in which the at least one organic layer exhibits different partial areas with various refractive indices. As a result of the deflection at the phase boundaries within the organic, fewer photons remain in the layer due to wave conduction losses than in homogenous layers.
Moreover, in addition to this utilization of intrinsic inhomogeneities in the active organic layer it is known that foreign bodies such as nanoparticles can be introduced into the organic electroluminescent material so that wave conductor effects within the organic can be avoided, see, e.g., “Enhanced luminance in polymer composite light emitting devices”, by S. A. Carter et al., Appl. Phys. Lett., vol. 71 (1997). These nanoparticles can consist, e.g., of TiO2, SiO2 or Al2O3 and be embedded in a polymeric emitter material such as MEH-PPV.
In addition to the bottom-emitting OLEDs the top-emitting OLEDs are becoming increasingly relevant since they have advantages over the first-named ones for specific applications. If both electrodes as well as the substrate are transparent a component can be made available that is electroluminescent in its totality, that is, that radiates up and down. If the substrate does not have to be transparent as in the top-emitting OLED, many other substrates can be used in addition to glass that make it possible, e.g., that the component is flexible, that is, bendable. Furthermore, even metal foils, silicon wafers or other substrates with silicon-based electronic components as well as printed circuit boards can serve as substrates in such a top-emitting electroluminescent component.